1. Field of the Invention
This invention relates to process and apparatus useful in catalytic gas synthesis reactions, and more specifically to process and apparatus useful in the synthesis of ammonia.
2. Description of the Prior Art
Generally, the manufacture of ammonia consists of preparing an ammonia synthesis gas from a nitrogen source, usually air, and from a hydrogen source, which is conventionally either coal, petroleum fractions, or natural gases. In the preparation of ammonia synthesis gas from natural gases, for example, a raw (that is, hydrogen-rich) synthesis gas is formed by first removing gaseous contaminants such as sulfur from the natural gas by hydrogenation and adsorption, and then by reforming the contaminant-free gas. The carbon monoxide in the raw synthesis gas is converted to carbon dioxide and additional hydrogen in one or more shift conversion vessels, and the carbon dioxide is removed by scrubbing. Further treatment of the raw synthesis gas by methanation may be used to remove additional carbon dioxide and carbon monoxide from the hydrogen rich gas, resulting subsequently in an ammonia synthesis gas containing approximately three parts of hydrogen and one part of nitrogen, that is, the 3:1 stoichiometric ratio of hydrogen to nitrogen in ammonia, plus small amounts of inerts such as methane, argon and helium. The ammonia synthesis gas is then converted to ammonia by passing the ammonia synthesis gas over a catalytic surface based on metallic iron (conventionally magnetite) which has been promoted with other metallic oxides, and allowing the ammonia to be synthesized according to the following exothermic reaction: EQU N.sub.2 +3H.sub.2 .fwdarw.2NH.sub.3
Ammonia synthesis, as is characteristic of exothermic chemical reactions, suffers from a competition between equilibrium and kinetics. The equilibrium conversion of hydrogen and nitrogen to ammonia is favored by low temperatures. However, the forward reaction rate to ammonia strongly increases with temperature. This leads to an optimal reactor temperature profile which starts relatively high, in order to get reaction rates as fast as possible while still far away from equilibrium, and which is then allowed to gradually fall along the reaction path in the reactor to improve equilibrium as the reaction progresses. Unfortunately, by definition, exothermic reactions give off heat, and hence the temperature tends to rise as the ammonia synthesis progresses, prematurely stopping the reaction when an unfavorable equilibrium is approached.
A number of solutions to this problem have evolved in the form of particular ammonia synthesis reactor designs. In modern, large scale ammonia plants (600 to 2,000 tons of ammonia per day) two general types predominate. Both use two or more adiabatic stages with cooling between stages in order to move away from equilibrium after each stage. The basic difference between the types of reactors is in the cooling method. In the first, a direct contact quench is used with a portion of unreacted cold feed being brought into contact with the heated effluent which is desired to be cooled. In the second type of reactor, indirect heat exchange is used to cool the desired gas streams. The former type of reactor is simpler in construction but is not as efficient because part of the feed by-passes all but the last stage in order to effect the desired cooling within the reactor. The optimum operation of either type, which can be readily calculated by one skilled in the art, employs a declining sequence of reaction stage outlet temperatures. This is illustrated by FIG. 7 of U.S. Pat. No. 4,181,701.
Since the reaction is exothermic, the heat of reaction can theoretically be recovered as useful waste heat. Conventionally, the waste heat is recovered from the reactor effluent, which, as previously mentioned, is relatively cold, since the last reaction stage has the lowest outlet temperature of the several beds within the reactor. Waste heat recovery between stages is known in the art and is disclosed in such references as U.S. Pat. Nos. 3,721,532; 4,101,281, 4,180,543, and 4,181,701 and in co-pending application Ser. No. 414,523 filed Sept. 2, 1982 (the disclosure of which application is hereby incorporated by reference). However, the reported schemes either require the expense of a second reactor vessel, or bear the risk of poisoning of the catalyst or of explosive and thereby safety-related problems in generating steam for removal of the reaction heat by use of steam generation coils located inside the reactor vessel, which generally contains a reduced catalyst that is potentially violently reactive with water or steam at the elevated temperatures which are used.